A solid oxide fuel cell (SOFC) is a device that generates electricity by a chemical reaction. FIG. 1 shows a conventional SOFC subcell including a cathode layer 102, an anode layer 106, and an electrolyte layer 104. Fuel cells are typically characterized by their electrolyte material, with SOFCs having a solid oxide or ceramic electrolyte.
During operation of the SOFC, an oxidant, usually air, is fed through a plurality of air channels 120 defined by the cathode 102, while fuel, such as hydrogen gas (H2), is fed through a plurality of fuel channels 121 defined by the anode 106. The oxidant and fuel channels can be oriented at right angles to one another. The anode and cathode layers are separated by an electrolyte layer 104. During operation, the oxidant is reduced to oxygen ions at the cathode. These oxygen ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons are transported through the anode to an external circuit (not shown) and then back to the cathode, providing a source of electrical energy in the external circuit.
The flow of electrons in the external circuit typically provides an electrical potential of approximately 1.1 volts. To generate larger voltages, fuel cells are typically arranged in “stacks” composed of a larger number of individual cells with an “interconnect” joining and conducting current between immediately adjacent cells. As described in greater detail below, the stack design shown in FIG. 2 is a flat-plate or “planar” SOFC, in which two separate “cells” are shown arranged in a repeating sequence. The cells are separated by an interconnect 208, 216 which serves to connect each cell in series so that the electricity each cell generates can be combined.
One continuing challenge in fuel cell manufacture is the prevention of gas leaks within and from the fuel cell. Gas leaks are problematic for several reasons. To attain a certain power, a stoichiometric ratio of oxygen to hydrogen greater than or equal to one is required. With a severe leak on the air side, there could be a superfluous amount of hydrogen relative to oxygen and performance of the fuel cell will suffer. Hydrogen or other fuel gas leaks can be even more significant because of the danger of explosion. Further, if the hydrogen leak comes into contact with the cathode material, the cathode itself may be permanently damaged by hydrogen reduction. This not only damages the electrical properties of the cathode layer, but can also cause a volumetric expansion (swelling) of the cathode layer that may result in complete failure of the stack. Lastly, leak of fuel or oxidant could decrease the fuel utilization or air utilization respectively.
Therefore, there is a need for an improved solid oxide fuel cell stack with reduced gas leakage.